# How to measure the angle between two functions

### Extending the definition of angles and orthogonality beyond Euclidean spaces

This will surprise you: *sine* and *cosine* are orthogonal to each other.

Even the notion of the enclosed angle between *sine* and *cosine* is unclear, let alone its exact value. How do we even define the angle between two functions? This is not a trivial matter. What’s intuitive for vectors in the Euclidean plane is a mystery for objects such as functions.

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